The invention relates to a system for recording/reading information on/from a disc. A disc cassette for a rotatable disc has outer case comprising main walls parallel to the disc, side walls, and a front opening which communicates with a disc-receiving space, a combined shutter and disc holder for closing the front opening of the case and movable latching means which are movable by external means from a latching position in which the shutter is latched in the case, to a free position in which the shutter can be slid out of the case; apparatus into which the disc cassette is introduced has release means for releasing the shutter of an inserted disc-cassette and a withdrawal mechanism for withdrawing the shutter and disc holder from the case and retaining the shutter and the disc in the apparatus during withdrawal of the case.
U.S. Pat. No. 4,164,782 (herewith incorporated by reference) discloses such a system intended for capacitively scanned video discs carrying information in the bottom of a groove in the surface of the disc. In view of the very fine structure of the grooves and the information, the disc must be protected adequately against dust and other extraneous influences. A ribbon-like element is attached to the shutter of the disc cassette and surrounds the disc with clearance to withdraw the disc from the case during removal of the case from the apparatus.
The movable latching means with which the shutter is latched in the case include two resilient plastics latching limbs which are integral with the plastics shutter and which are disposed on one side of the disc. At their free ends the latching limbs each carry a latching projection, which projections engage in corresponding recesses in the inner side of the sleeve when the shutter is disposed on the case to retain the shutter in the case. The latching limbs are movable from the latching position to the free position by the release means of the apparatus which at the location of the front opening are in contact with the free ends of the latching limbs to deflect these limbs to such an extent that the latching projections are disengaged from the corresponding recesses, thus enabling the shutter to be withdrawn from the case. The shutter has a central recess with stops at both sides to cooperate with the withdrawal mechanism of the apparatus. The relevant parts of the withdrawal mechanism engage behind the stops, so that once the loaded disc cassette has been inserted fully, the latching limbs are moved to the free position by the release means of the apparatus and at the same time the withdrawal mechanism cooperates with the stops in the centre of the shutter. Withdrawal of the case results in the shutter together with the video disc being withdrawn from the case.
In the known system the resilient latching means form part of the shutter. If the latching limbs should perform their function properly, also in the long run, they must be comparatively long and/or thin to avoid excessive material stresses. A great length of the limbs is undesirable because this results in larger dimensions of the shutter and, consequently, the disc cassette. Thin latching limbs have the drawback that they are vulnerable and weak and therefore in particular when they are made of a plastics, cannot guarantee a safe and reliable long-term storage of the heavy video disc, which has a diameter of approximately 30 cm.
Relaxation effects often occur in the case of prolonged loading of a plastic spring, which under load a plastics spring tends to deform lastingly with a consequent reduction in resilience. After removal of the load the spring fails to resume its origin al position, although in many cases it is possible, depending on the type of plastic used, that in time the spring ultimately resumes its original position.
In particular in professional systems, where the disc cassette contains for example a professional optical disc, it is not unlikely for the disc cassette to remain in the apparatus for a longer period. The system may form part, of the peripheral equipment of a computer and the disc may be provided with a fixed computer-readable program. For such uses it is therefore unfavourable if in the long run the latching performance of the shutter latching means should degrade owing to relaxation effects, which increase with increasing temperature. However, the electronic circuitry and electric motors and transformers used in the apparatus inevitably raise the temperature in such an apparatus higher than room temperature and not unlikely to 70.degree. C. or higher.
It has been proposed to provide the apparatus with a release mechanism of a type which does not permanently load, or which at least loads only moderately, the latching means of the disc cassette once the case has been withdrawn from the apparatus U.S. Pat. No. 4,239,238, herewith incorporated by reference, describes a system in which the Patent Specification the release means in the apparatus, after removal of the case, are moved by the latching means of the shutter itself, opposed by a moderate spring force, to a position in which the latching limbs are subjected only to moderate spring force. In a known video-disc system the plastic latching limbs for latching the shutter in the disc cassette are replaced by metal latching limbs. The metal latching limbs are embedded in the plastic material of the shutter and carry plastic projections at their free ends. This solution precludes or at least mitigates the relaxation problem, but also leads to an undesired increase in the number of parts and to complications and increased cost of the production process. Moreover, in view of the requirements to be met by the spring material, including freedom from corrosion, this means that a comparatively expensive material such as phosphor bronze must be chosen.